This invention relates to the manufacture of discs suitable for the recording and reproduction of information including video information. More particularly, this invention relates to a method of preparing smooth grooves on the surface of a disc prior to recording such information thereon.
A system for recording and playback of video information has been described in a copending application Ser. No. 126,772, of Jon K. Clemens, filed Mar. 22, 1971, now U.S. Pat. No. 3,842,194, issued Oct. 15, 1974. According to this system, a lacquered surface is deposited on a thick aluminum disc and a continuous spiral groove is cut into the lacquered surface. A first nickel replica is then made of the grooved lacquer surface by depositing nickel over such surface and thereafter separating the nickel coating from the lacquer. This first nickel replica is a negative reproduction of the original grooved lacquer. A second nickel replica is made of the first replica to form a metal disc that is a positive reproduction of the original grooved lacquer. A coating of electron beam sensitive material, for example, Shipley No. 1350 Photoresist manufactured by Shipley Company of Newton, Massachusetts, is thereafter applied to the second nickel replica by a process such as, for example, the one described in a copending application Ser. No. 245,657 in the names of Robert Michael Mehalso and David Isaac Harris entitled, "IMPROVEMENT IN THE MANUFACTURE OF VIDEO DISC" and assigned to RCA Corporation, now U.S. Pat. No. 3,795,534, issued Mar. 5, 1974.
The photoresist-coated replica is then exposed to a video-signal-modulated beam of a scanning electron microscope, which provides exposure of the photoresist in the groove of the disc in correspondence to the video signal information. The photoresist is then developed and the exposed portions of photoresist are removed to form a topography in the groove corresponding to the video signal information. A nickel replication is made of the resultant disc, and this replication is utilized to stamp or emboss vinyl records by techniques known in the audio recording art. The vinyl replica is then metalized to make the surface conducting, and the metalization is thereafter coated with a dielectric. In playing back the recorded information, a stylus is caused to ride in the dielectric coated groove. This stylus, along with the metalization and dielectric, acts as a capacitor. Capacitance variations in the groove, which correspond to the recorded video information, are then detected electronically to recover the video information.
In order to produce images in the playback operation which are relatively free of background noise, it is necessary to eliminate the transfer to the final vinyl record of irregularities formed in the original grooved lacquer.
In practice, the cutting of a smooth groove in the original lacquer, free of irregularities, has been a severe problem. The spiral groove is cut by a sharp cutting stylus of a predetermined shape (such as triangular cross-section) into the lacquer layer on the metal disc. As cutting proceeds, irregularities form in the peak regions between adjacent convolutions of the groove, the peak regions being known in the audio recording art as land regions. These irregularities, unless masked or removed, are replicated in the vinyl record. Subsequently, when the vinyl record is played back, the embossed irregularities in the peak regions of the groove perturb the playback stylus and cause erroneous signal (noise) pickup.